Very little is currently known about the molecular mechanisms of protein aggregation. The increasing public concern about protein deposition diseases, both systemic and neurodegenerative, emphasize the growing recognition of the critical importance of protein aggregation and makes this a very important area for study. This proposal is aimed at elucidating the detailed molecular mechanisms of protein aggregation that lead to ordered aggregates such as amyloid fibrils, as well as disordered or amorphous aggregates such as inclusion bodies. Understanding the mechanisms of deposition will facilitate the rational design of clinical therapies that are based on inhibition of protein aggregation. There is a critical need for developing therapeutic solutions to protein deposition diseases. For this proposal we have chosen to focus on one particular protein system, the variable domain of the immunoglobulin light chain, VL, which has a number of features which make it well-suited for our purposes. Each year thousands of people die in the US from several types of light chain deposition disease. These disorders pose challenging biophysical questions regarding the relationship between protein stability, folding and aggregation. We hypothesize that partially-folded intermediates are the critical precursors to protein aggregation. The specific aims of the proposal are (1) to determine why some proteins have a strong propensity to aggregate, and why some form amyloid and others amorphous aggregates, (2) to determine the mechanisms of protein aggregation, especially amyloid fibrils, and (3) to identify inhibitors of light chain aggregation and fibril formation, which could lead to the development of molecules of potential therapeutic value. Techniques to be used include mutagenesis, chromatography, kinetics, ATR FTIR spectroscopy, atomic force and electron microscopy, and other biophysical/biochemical methods.